Carbon fibers are lightweight and excellent in strength and elastic modulus, and thus are combined with various matrix resins to form composite materials, which are used in various fields including aircraft members, spacecraft members, automobile members, ship members, constructional materials, and sporting goods. In order to impart excellent characteristics of carbon fibers to a composite material including the carbon fibers, excellent adhesion between the carbon fibers and a matrix resin is important.
In order to improve the adhesion between carbon fibers and a matrix resin, the carbon fibers are typically subjected to oxidation such as gas phase oxidation and liquid phase oxidation, and thus oxygen-containing functional groups are introduced to the surface of the carbon fibers. For example, a disclosed method includes subjecting carbon fibers to electrolysis to improve interlaminar shear strength as an index of the adhesion (see Patent Literature 1). However, as a composite material has been required to have higher characteristics in recent years, the adhesion achieved by such an oxidation alone is becoming insufficient.
Carbon fibers are brittle and poor in bindability and abrasion resistance and thus readily generate fluffs or broken threads in a high-order processing step. To address this issue, methods of coating carbon fibers have been disclosed (see Patent Literatures 2 and 3).
For example, a disclosed sizing agent is an aliphatic compound having a plurality of epoxy groups (see Patent Literatures 4, 5, and 6). Separately, a disclosed method includes applying, as a sizing agent, an epoxy adduct of polyalkylene glycol onto carbon fibers (see Patent Literatures 7, 8, and 9).
Another disclosed method includes applying, as an aromatic sizing agent, a diglycidyl ether of bisphenol A onto carbon fibers (see Patent Literatures 2 and 3). Another disclosed method includes applying, as a sizing agent, a polyalkylene oxide adduct of bisphenol A onto carbon fibers (see Patent Literatures 10 and 11). Another disclosed method includes applying, onto carbon fibers, a sizing agent that is obtained by adding an epoxy group to a polyalkylene oxide adduct of bisphenol A (see Patent Literatures 12 and 13).
Although the sizing agents above can impart adhesiveness and bindability to carbon fibers, a sizing agent including one epoxy compound is insufficient, and thus, in recent years, a technique of using two or more epoxy compounds in combination has been developed depending on an intended function.
For example, a disclosed sizing agent includes two or more epoxy compounds each having a defined surface free energy (see Patent Literatures 14 to 17). Patent Literature 14 discloses a combination of an aliphatic epoxy compound and an aromatic epoxy compound. Patent Literature 14 describes that a sizing agent present in the outer layer in a large amount has an effect of shielding another sizing agent present in the inner layer in a large amount from air, and this prevents the epoxy group form undergoing ring-opening by water in air. Patent Literature 14 also describes that the sizing agent preferably contains the aliphatic epoxy compound and the aromatic epoxy compound in a ratio of 10/90 to 40/60, and the aromatic epoxy compound is preferably contained in a larger amount.
Patent Literatures 16 and 17 disclose a sizing agent including two or more epoxy compounds having different surface free energies. Patent Literatures 16 and 17 mainly aim to improve permeability of a matrix resin by the sizing agent including two or more epoxy compounds having different surface free energies, but do not limit the combination of two or more epoxy compounds to the combination of an aromatic epoxy compound and an aliphatic epoxy compound, and describe no typical example of the aliphatic epoxy compound selected in view of adhesion.
Another disclosed sizing agent contains a bisphenol A epoxy compound and an aliphatic polyepoxy resin in a mass ratio of 50/50 to 90/10 (see Patent Literature 18). However, the sizing agent disclosed in Patent Literature 18 also contains the bisphenol A epoxy compound as an aromatic epoxy compound in a large amount.
A disclosed sizing agent specifying the combination of an aromatic epoxy compound and an aliphatic epoxy compound is a combination of a multifunctional aliphatic compound on the surface of carbon fiber bundles and an epoxy resin, a condensate of an alkylene oxide adduct with an unsaturated dibasic acid, and an alkylene oxide adduct of a phenol on the surface of the multifunctional aliphatic compound (see Patent Literature 19).
A disclosed combination of two or more epoxy compounds is a combination of an aliphatic epoxy compound and a bisphenol A epoxy compound as an aromatic epoxy compound. The aliphatic epoxy compound is a cyclic aliphatic epoxy compound and/or a long chain aliphatic epoxy compound (see Patent Literature 20).
A combination of epoxy compounds having different properties has also been disclosed. A disclosed combination contains two epoxy compounds that are liquid and solid at 25° C. (see Patent Literature 21). Furthermore, a combination of epoxy resins having different molecular weights and a combination of a monofunctional aliphatic epoxy compound and an epoxy resin have been developed (see Patent Literatures 22 and 23).
However, the sizing agents (for example, Patent Literatures 20 to 23) containing two or more components practically fail to achieve both the adhesion and the stability of a prepreg during long-term storage. The reason is considered as follows: The following three requirements are needed to be satisfied in order to simultaneously achieve the high adhesion and the suppression of the reduction in physical properties of a prepreg during long-term storage, but a conventional combination of any epoxy resins has failed to satisfy these requirements. Of the tree requirements, the first is that an epoxy component having high adhesion is present in the inner side (carbon fiber side) of a sizing layer, and the carbon fibers and the epoxy compound in the sizing interact strongly; the second is that the surface layer (matrix resin side) of the sizing layer has a function of suppressing the reaction between a matrix resin and the epoxy compound that is present in the inner layer and that has high adhesion to carbon fibers; and the third is that the surface layer (matrix resin side) of the sizing agent necessitates a chemical composition capable of strongly interacting with a matrix resin in order to improve the adhesion to the matrix resin.
For example, Patent Literature 14 discloses a sizing agent having an inclined structure for increasing the adhesion between carbon fibers and the sizing agent, but Patent Literature 14 and any other literatures (for example, Patent Literatures 15 to 18) have no idea that the sizing layer surface simultaneously suppresses the reaction between an epoxy compound having high adhesion to carbon fibers and a component in a matrix and achieves high adhesion to the matrix resin.
Patent Literature 19 discloses a sizing agent including an inner layer containing a multifunctional aliphatic compound and an outer layer containing an aromatic epoxy resin and an aromatic reaction product each having low reactivity. The sizing agent should prevent a prepreg stored for a long period of time from suffering change with time, but the surface layer of the sizing agent contains no multifunctional aliphatic compound having high adhesion, and this makes it difficult to achieve high adhesion to a matrix resin.